G.I.S. for the protection and management of Cultural Heritage; and (6) Signiicance,
This volume publishes a total of seventy-two contributions which relect some of application of different scientiic approaches to the common goal of the conservation
Science, Technology
and
Cultural Heritage
editor: M.A. Rogerio-Candelera
SCIENCE, TECHNOLOGY AND CULTURAL HERITAGE
PROCEEDINGS OF THE SECOND INTERNATIONAL CONGRESS ON SCIENCE AND TECHNOLOGY FOR THE CONSERVATION OF CULTURAL HERITAGE, SEVILLA, SPAIN, 24–27 JUNE 2014
Science, Technology and Cultural Heritage
Editor Miguel Ángel Rogerio-Candelera Instituto de Recursos Naturales y Agrobiología de Sevilla (IRNAS-CSIC), Seville, Spain
CRC Press/Balkema is an imprint of the Taylor & Francis Group, an informa business © 2014 Taylor & Francis Group, London, UK Typeset by V Publishing Solutions Pvt Ltd., Chennai, India Printed and bound in Great Britain by CPI Group (UK) Ltd, Croydon, CR0 4YY All rights reserved. No part of this publication or the information contained herein may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, by photocopying, recording or otherwise, without written prior permission from the publisher. Although all care is taken to ensure integrity and the quality of this publication and the information herein, no responsibility is assumed by the publishers nor the author for any damage to the property or persons as a result of operation or use of this publication and/or the information contained herein. Published by: CRC Press/Balkema P.O. Box 11320, 2301 EH Leiden, The Netherlands e-mail:
[email protected] www.crcpress.com – www.taylorandfrancis.com ISBN: 978-1-138-02744-2 (Hbk) ISBN: 978-1-315-71242-0 (eBook PDF)
Science, Technology and Cultural Heritage – Rogerio-Candelera (Ed) © 2014 Taylor & Francis Group, London, ISBN 978-1-138-02744-2
Table of contents
Science, Technology, and Cultural Heritage: An inexorable relationship M.A. Rogerio-Candelera
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Climate change, sea level rise and impact on monuments in Venice D. Camuffo, C. Bertolin & P. Schenal
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Air pollution and preventive conservation in some European museums R. Van Grieken Low cost strategies for the environmental monitoring of Cultural Heritage: Preliminary data from the crypt of St. Francesco d’Assisi, Irsina (Basilicata, Southern Italy) M. Sileo, M. Biscione, F.T. Gizzi, N. Masini & M.I. Martinez-Garrido Monitoring moisture distribution on stone and masonry walls M.I. Martinez-Garrido, M. Gomez-Heras, R. Fort & M.J. Varas-Muriel Effects of open shelters on limestone decay: The case-study of the Bishop’s Palace archaeological site in Witney (England) C. Cabello Briones Air quality assessment and protection treatments impact on the collection of the Museo Naval (Madrid, Spain) J. Peña-Poza, F. Agua, J.F. Conde, P. De San Pío, S. García Ramírez, J.M. Gálvez Farfán, J.M. Moreno Martín, M. González Rodrigo, M. García-Heras & M.A. Villegas Establishing the relationship between underwater cultural heritage deterioration and marine environmental factors. A comparative analysis of the Bucentaure and Fougueux sites T. Fernández-Montblanc, M. Bethencourt, A. Izquierdo & M.M. González-Duarte Natural gamma radioactivity in granites with different weathering degrees: A case study in Braga (NW Portugal) M. Lima, C. Alves & J. Sanjurjo
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27 35
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Accelerated weathering test as environmental behaviour trials on metals M.A. Gómez-Morón, F. Martín-Cobos & P. Ortiz
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Painting woods vulnerability to ultraviolet exposure M.A. Gómez-Morón, A. Tirado & P. Ortiz
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Physical characterization of super-fragile materials in underwater archaeological sites L.C. Zambrano, M. Bethencourt & M.L.A. Gil
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Underwater Cultural Heritage risk assessment related to mean and extreme storm events: A modelling case study in the Bay of Cadiz T. Fernández-Montblanc, A. Izquierdo & M. Bethencourt
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Another source of soluble salts in urban environments due to recent social behaviour pattern in historical centres B. Cámara, M. Álvarez de Buergo, R. Fort, C. Ascaso, A. de los Ríos & M. Gomez-Heras
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Science, Technology and Cultural Heritage – Rogelio Candelera (Ed) © 2014 Taylor & Francis Group, London, ISBN 978‐1‐138‐02744‐2
Underwater cultural heritage risk assessment related to mean and extreme storm events: A modelling case study in the Bay of Cadiz T. Fernandez-Montblanc University of Cadiz, Marine Science and Technological Center of Andalusia, International Campus of Excellence of the Sea (CEI∙MAR), Puerto Real, Cádiz, Spain
A. Izquierdo University of Cadiz, Marine Science and Technological Center of Andalusia, International Campus of Excellence of the Sea (CEI∙MAR), Puerto Real, Cádiz, Spain
M. Bethencourt University of Cadiz, Marine Science and Technological Center of Andalusia, International Campus of Excellence of the Sea (CEI∙MAR), Puerto Real, Cádiz, Spain
ABSTRACT: Underwater cultural heritage (UCH) risk assessment due to mean and extreme storms was conducted at Bucentaure and Fougueux shipwreck sites. Storms representative of mean and extreme conditions were simulated using numerical modeling tools. Temporal series of sea level, waves, fraction breaking and maximum bed shear stress were obtained at the studied sites. The type of sediment transport and the hypothetical critical size of an archaeological object to be moved was established. From our results it may be inferred that mean and extreme storms are a greater threat for UCH at the Fougueux site.
1 INTRODUCTION In situ protection option has been included amongst the main principles of the UNESCO Convention on the Protection of the Underwater Cultural Heritage 2001 (UNESCO, 2002: 53-66). In addition, remarkable advances in acoustic seafloor mapping during the past two decades have promoted the discovering of a large number of shipwreck sites. This fact along with the costs related to protection make it necessary to develop tools and methodologies to identify the sites more susceptible to be damaged, which require special safeguard measures. This methodology should include, firstly, the valuation of the magnitude of the possible loss or damage, in other words the significance levels of the underwater archaeological sites in terms of archaeological value. Secondly, the identification of the hazards that could damage the underwater archaeological sites, and the occurrence probability. The later is a difficult task, takeing into account the complexity of processes in the marine environment. The deterioration and the degree of protection required for shipwrecks may change in time as a consequence of variations in physicochemical environmental conditions, burial exposure events, changes in biological community and anthropogenic disturbances (Gregory et al., 2012). The relative contribution of each factor to the degradation process will vary widely depending on the existing conditions at the UCH site. However, physical process may be considered as key in the UCH degradation in later stages, when the archaeological site has reached a quasi-equilibrium state. Especially in shallow water areas, the hydrodynamic and morphodynamic conditions existing during mean and extreme storm events can be considered a key factor in the degradation and in the remains scattering. Taking into account the crucial role of hydrodynamics and geomorphology in shallow waters, during storm events, waves can be considered as a major hazard to the UCH. On one hand, waves physically impact on the sites, causing damage on archaeological remains, as well as UCH descontextualization produced by the dispersion of the artifacts. Furthermore, geomorphologic changes and sediment transport induced by waves can exert control over other factors. In the case of chemical factors, burial or exposition caused by sediment dynamics may create oxic or anoxic conditions. In addition, erosion or deposition around shipwrecks may modify the biological community, or the thickness of the calcareous concretion layer. The later is developed
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around the submerged objects and acts as a protective layer against the chemical and mechanical degradation. Thus, the aim of this work is to assess the storm risk that threats the two different shipwrecks under study (Fougueux and Bucentaure Sites). For this purpose a coupled hydrodynamic and wave propagation model was implemented in the Bay of Cadiz. The obtained wave and current fields were used for the estimation of the forces affecting the deterioration and scattering of archaeological remains. 2 MATERIALS AND METHODS 2.1 Study area The study area is located in the vicinities of Cadiz bay in the southwest coast of the Iberian Peninsula (Fig. 1.a). Two shipwrecks under study are associated with Fougueux and Bucenaure ships, two war ships belonging to the combined franco-spanish fleet in the Battle of Trafalgar (1805). They were sunk during the violent storm that struck the coast of Cádiz after the Battle of Trafalgar. The ship S.M.I. Bucentaure, sank on October 23, represents a scattered shipwreck site with a total of 22 iron guns, remains of an anchor. Seated at 12 m depth, is located in the outer Bay of Cadiz (Fig. 1.b). Meanwhile, S.M.I. Fouguex, sunk on October 22, 1805, preserves an important portion of the hull structure along with 25 cannons and an anchor (Fig. 1.c). Fougueux is seated at 7 m depth in front of Sanctipetri sandspit. The seabed at the Fouguex site is mainly composed of unconsolidated sediments, medium and fine quartz sands (D50=176.8 mm). In the case of Bucentaure, the site seafloor has a combined composition, with rocky seabed and gravelly quartz sand (D50=1.095 mm). At the vicinities of the Fougueux study area the bathymetry shows an alignment of isobaths parallel to the coastline, oriented NNW- SSE. At the Bucentaure site, isobaths have a WSW-ENE orientation, parallel to south side of the outer bay, which may promote wave refraction. West and WSW wind waves are the main hydrodynamic agent in the area of interest, according to the data recorded at the Cadiz buoy. During stormy weather significant wave height (Hs) exceeds 4 m, although only for 90 hours a year in average. Therefore the area can be classified as a low-energy coast (Benavente et al., 2000). Regarding the tidal range, the study area can be classified as meso-tidal coast (mean spring range 2.96 m).
Figure 1. (a) Location of studied sites. (b) Bathymetry of Bucenture site. (c) Bathymetry of Fougeuux taure site.
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2.2 Hydrodynamic model Monitoring waves and currents is expensive. Moreover the information is oftenly restricted to single observation points. However, computational fluid dynamic models allow us, after calibration and validation process, to know wave and current fields, and other derived parameters in the whole domain. In this case, for the hydrodynamic modeling the Delft3D-FLOW model was used (Lesser et al., 2004), that, for the purposes of this study, included wave, currents and water level. Delft3D uses SWAN (Booij et al., 1999), a phase-averaged wave model,. In order to model the hydrodynamic conditions wave and flow modules were coupled, which allows the inclusion of wave-induced currents, changes in water levels, and wave-current interaction effects. The flow model setup was carried out using a 2D depth averaged approach, taking into account that the interest areas are located in well-mixed open sea waters. The modeling area extends from Trafagar Cape to Chipiona Coast. In an coarse grid with 252 m resolution were successively nested three finer grids allowing 9 m final resolution on the study areas. The tidal boundary conditions of the model included 5 main tidal constituents in the area (i.e. O1, K1, K2, N2, S2, M2) taken from the model AG95.1 (Andersen, 1995). The wave model setup was conducted using a fine model grid (10m resolution) in the shipwreck sites nested to a coarser 50m resolution grid. Wave open boundary conditions were set as constant, and taken from wave directional buoy data pertaining to REDCOS network of Spanish agency, Puertos del Estado (Fig 1.a). In addition, to ensure a good reproduction of the real hydrodynamics by the model, a calibration and validation procedures were carried out, comparing experimental in situ wave, current and water level data (Fig. 1.a) with the model output. Finally, as a typical storm event, a storm occurring in December 2009 was simulated. The storm may be divided into two phases. The first phase represents typical mean storm conditions, with observed significant wave height (Hs) up to 2 m , 9 s peak period (Tp) from WSW direction. The second phase represents extreme stormconditions with Hs=4.8 m, Tp=10 and SW direction. In this way time series of waves, current, and other variables related with sediment dynamics and wave exerted forces (maximum bed shear stress, fraction breaking ) were obtained in both study sites. 2.3 Sediment transport and stability When and object is placed on the seabed, the flow speed up around it, so under energetic conditions imposed by current and waves, more sediment is carried away around the objects that is carried in the vicinity of the object (Soulsby, 1997). These may cause scouring process around shipwrecks sites or artifacts. In this way, material which was buried can be now exposed, changing anoxic to oxic conditions. Furthermore, if sediment is stirred up and transported in suspension mode, an increase in scouring occurs. This fact can promote an increase in chemical and biological degradation of different materials. Moreover, when sediment is transported in suspension mode, it may affect the UCH conservation due to the sediment abrasive effect, impacting directly on materials or degrading the concretion layer that protects them. Another possible damage at underwater archaeological site is the scattering and loss of objects and artifacts due to the drag forces induced by waves and currents. Therefore, for a first threat estimation two criteria (suspension threshold and critic size of object for movement) were calculated with the model output obtained for the simulated storm, as follow: Suspension criteria: transport in suspension occur when settling velocity (w) is smaller than skin-friction velocity (u*); w / u* < 1
w u *
1 1 g 3 s 10.36 2 1.049 d d 2
3
0.5
0.5 max 10.36 /
Critical size for object movement (assuming spherical shape and quartz density):
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(1)
3.08 97.9U w Dcr T 1.08 g 1 2.08 s
0.5
(2)
Where, ν = kinematic viscosity; d: grain size diameter; ρ = water density; ρs = sand density; τmax = maximum bed shear stress; Uw = wave orbital velocity at seabed; T = wave period. Obviously, critical size depends on shape and density of the object, for simplicity spherical shape and quartz density were the assumed values.
Figure 2. Model output at Fougueux (F) and Bucentaure (B) points. (a) Depth. (b) Significant wave height recorded in a buoy and used like input, and values recorded in studied sites. (c) Fraction of wave breaking. (d) Maximum bed shear stress due to wave + current enhancement.
3 RESULTS Figure 2 show the time series in both studies sites obtained through model output. Significant differences are found in the wave storm effect on the study sites. In Figure 2.a depth variation due to tidal level changes was plotted. Depth variation represents 25% in case of Fougueux and 14% in Bucentuare site. This fact induced greater changes in wave forces exerted on Fougueux site. Figure 2.b shows the time series of observed Hs used as model input, and model output in both sites. Regarding to mean storm conditions, until 21th December, 2.2-1.4 m Hs was registered. At Fougueux Hs always was 0.2-0.4 m greater than that simulated at Bucentaure site. Wave shoaling process enlarges Hs in Fouguex site with respect to input data, whereas diffraction and refraction process reduce Hs at Bucentaure site. Concerning to extreme storm conditions, Hs varied between 3 and 4.2m. Hs values were 0.2- 0.5 m grater in Fougueux site, with the exception of periods in which input data values were greater than 4m and low water occurred simultaneously. This was due to wave breaking limitation imposed by depth during low water. Such as Figure 3.b shows, wave breaking process solely occur at Fougueux site during extreme storm phase. When coupled with tidal cycles, the fraction breaking increase up to 0.010.02 during low water.
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Maximum bed shear stress (τmax) is the maximum friction force exerted on the seabed during the wind wave cycle. It was calculated taking into account wave and current friction forces. Figure 4.d shows a great difference in τmax between both sites. This difference is mainly correlated with the reduced depth at Fugueux site. During the mean storm phase, the values of τmax were close to 50 N m-2 and 20 N m-2 at Fougueux and Bucentuare sites, respectively. Larger differences were found during the extreme storm phase. Thus, at Fougueux site τmax mean value was 170 N m-2, with a maximum value of 220 N m-2 reached when maximum Hs matches with high water. However at Bucentaure site τmax mean value was 75 N m-2 , reaching a maximum of 132N m-2. Sediment transport criterion and object stability plotted in Figure 3.a show significant dissimilitude in both sites. During the mean storm, the w / u* ratio was nearly 0.5 at the Fouguex site, unlike values close to 1 reached at Bucentaure. As a result, during mean storm phase, sediment transport in suspension mode mostly takes place at Fouguex site. During extreme storm phase w / u* values were lower than unity at both sites, highlighting that sediment transport occur in suspension mode. Although, still great differences exist, w / u* ratio was around 0.5 at Bucentaure site, whereas at Fouguex was 0.25. It should be noted that values in Fouguex site during mean storm, a much more frequent condition, reached the same value that those register at Bucentaure site during extreme conditions. The results agree with the bathymetric data showed in Figure 1. At Fougueux site (Fig. 1.b) a large scour hole 0.8 m depth and 20 m wide was found, whereas at Bucentaure (Fig. 1.c) site no scouring was observed. On the other hand, results are also consistent with the measurements of the thickness of calcareous concretion layer developed around cannons. In this case, neglecting other factors, the abrasive effect caused by sediment on calcareous concretion layer seems greater at Fougueux site, where the average thickness measured was 5mm, while in Bucentaure cannons the concretion thickness was 20 mm . Regarding the critical size of objects that can be moved at both sites, remarkable dissimilarities were found, such as shown in Figure 3.b. Assuming the above outlined shape and density, only little objects smaller than 0.05 m diameter can be displaced and transported in both sites during mean storm conditions. However, during extreme storm phase, objects and artifacts up to 0.22 m diameter may be displaced at the Fougueux site. In opposition, the maximum size of a moveable object in Bucentaure was only 0.07 m. It should be noted, the wave breaking effects were omitted in the calculation, which may induce the underestimation of the forces exerted and the moveable object size at Fougueux. 4 CONCLUSION The modeling study carried out in the bay of Cadiz had allowed the risk assessment of mean and extreme storm events on Fougeux and Bucentaure sites. Model results show significant differences between the studied areas. The Fougueux site is exposed to a more energetic wave regime. Thus, mean and extreme storm suppose a greater threat to Fouguex site due to the following reasons. First, the sediment transport in suspension mode induces greater scouring process. This implies changes in chemical conditions that can endanger the archaeological remains. Second, sediment suspension transport increases the abrasive effect of sand on archaeological remains. Finally, the greater forces exerted by waves in Fougeux site can promote scattering and loss of archaeological objects. Modeling studied like the one presented here can be used in larger areas, allowing in an inexpensive and easy way the UCH risk assessment related to wave storm events. In addition, the results obtained in modeling studies can also be used to test and evaluate the effectiveness of the protection measures against storm waves. In conclusion, the combination of modelling studies and underwater archaeological mapping can be valuable for the management of underwater archaeological sites and to priorize and design the in situ protection projects. Therefore, underwater archaeological heritage managers may find modeling a useful tool for implementing the in situ protection guidelines provided by UNESCO in 2001.
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Figure 3. (a) Criterion for the threshold of suspension of sediment. Fall velocity (w) maximum skin friction (u*) ratio. (b) Critical diameter of object mobility, assuming spherical shape and quartz density.
ACKNOWLEDGEMENTS This study has been supported by project ARQUEOMONITOR CTM2010- 16363 (Spanish Ministry of Economy and Competitiveness).
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